The human auditory processing system segregates sound objects from complex auditory scenes using several binaural cues such as interaural time and level differences (ITD/ILD) and monaural cues such as harmonicity or common onset. This process is known as auditory scene analysis (ASA) as described more fully in A. S. Bregman Auditory Scene Analysis: The Perceptual Organization of Sound, MIT Press, Cambridge, Mass. (1990), incorporated herein by reference.
Hearing impaired patients have difficulties successfully performing such an auditory scene analysis even with a hearing prosthesis such as a conventional hearing aid, a middle-ear prosthesis, a bone-anchored hearing prosthesis, a cochlear implant (CI), or an auditory brainstem implant (ABI). Cues such as harmonicity, which the normal human auditory processing system uses for ASA, are not correctly reproduced by the current cochlear implants and auditory brainstem implants. This is especially a problem for audio recordings and live audio streaming. Processing methods such as directional microphones or steerable beamforming do not help hearing prostheses handle audio recordings played with standard sound systems, (i.e. stereo loudspeakers or headphones) because such techniques require true spatial sound sources.
Because of such problems, hearing aid users often are unable to listen to a single individual sound source within a mixture of multiple sound sources. In the case of understanding speech, this translates into reduced speech intelligibility. In the case of music, musical perception is degraded due to the inability to successfully isolate and follow individual instruments.
To assist cochlear implant users in performing an auditory scene analysis, an alteration of the sound signals is normally applied that emphasizes the sound sources of interest; see e.g., U.S. Pat. No. 8,369,958, which is incorporated herein by reference.